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Manuscript Title: Elastic scattering of electrons and positrons by atoms. Schrodinger and Dirac partial wave analysis.
Authors: F. Salvat, R. Mayol
Program title: PWASCH AND PWADIR
Catalogue identifier: ACJX_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 74(1993)358
Programming language: Fortran.
Computer: IBM AT 486/33.
Operating system: MS-DOS Rel. 5.0.
RAM: 370K words
Word size: 8
Keywords: Atomic physics, Electron, Elastic, Scattering, Static field Approximation, Partial-wave analysis, Schrodinger phase shifts, Dirac phase shifts, Monte carlo simulation.
Classification: 2.4.

Subprograms used:
Cat Id Title Reference
ABTR_v1_0 RADWEQ CPC 62(1991)65

Revision history:
Type Tit le Reference
correction 000A CORRECTION 08/08/96 See below

Nature of problem:
These codes provide a complete description of elastic scattering of electrons and positrons by neutral atoms. They use the static field approximation with non-relativistic (Schrodinger) and relativistic (Dirac) partial wave analysis. Scattering of other particles by finite- range central fields can be dealt with by simply expressing all input lengths and energies in generalized atomic units (m=n=e=1). The codes also give the percentage points of the single scattering angular distribution required for Monte Carlo simulations of multiple scattering processes. Results are delivered to the main program, or printed in the output file, after calling a single subroutine. The scattering field may be read from the input file or directly computed from an analytical approximation to the Dirac-Hartree-Fock-Slater field which is included in the programs.

Solution method:
The scattering field is approximated by a cubic spline, whose knots are dense enough to make sure that interpolation errors are negligible. The radial wave equations are solved by using the power series method due to Buhring [Z. Phys 187(1965)180] as implemented in the RADWEQ subroutine package [Comput. Phys. Commun 62(1991)65]. Particular techniques to speed up the convergence of the Legendre series, and to reduce the number of directly computed phase shifts, are adopted. The user can control the accuracy of the computed phase shifts through the parameter epsilon, which governs the global accuracy of the RADWEQ subroutines.

Phase shifts are obtained by matching the computed radial wave function with the free-particle wave function, i.e. a combination of spherical Bessel and Neumann functions. As a consequence, the codes work only for scattering by finite-range fields. They properly handle electron and positron scattering by neutral atoms, but not scattering by ions.

Unusual features:
The adopted RADWEQ routines offer the possibility of selecting the accuracy of calculated radial wave functions and phase shifts. When using optimum accuracy (i.e. epsilon~=10**-15 with double precision arithmetic) truncation errors, inherent to most numerical integration methods, are effectively eliminated. The PWADIR code gives reliable results for electrons and positrons with kinetic energies from ~1 keV to ~1 MeV.

Running time:
For a given problem, the running time increases when the desired accuracy is increased (by descreasing epsilon). The complete calculation for scattering of 10keV electrons by free A1 atoms using the PWADIR (PWASCH) program with epsilon=10**-8 takes 110 (45) seconds on the AT, 14 (6) seconds on the HP 900/720, 470 (201) seconds on the VAX 6310 and 25(11) seconds on the IBM 3090/600.

Manuscript Title: Unpublished correction to elastic scattering of electrons and positrons by atoms. Schrodinger and Dirac partial wave analysis. (C. P.C. 74(1993)358).
Authors: F. Salvat, R. Mayol
Program title: 000A CORRECTION 08/08/96
Catalogue identifier: ACJX_v1_0
Distribution format: gz
Classification: 2.4.